Lubrication system for a tool with a combustion engine

ABSTRACT

A tool consists of a combination engine in the form of a two-cycle engine, and a motion conversion device which is connected to said combustion engine and which is located in a housing. Said housing is filled with oil for lubricating the motion conversion device. A conveying device with which the oil is conveyed from the housing to the combustion engine is provided, said combustion engine being lubricated exclusively with oil from the housing and having no separate oil supply.

[0001] The invention relates to a tool in accordance with the preambleof claim 1.

[0002] Tools, in particular hand-held tools, such as e.g. a rammer forground compaction purposes, which comprise a combustion engine as thedrive are known in various forms. Two-cycle engines have proven to beparticularly suitable as combustion engines and have a high specificoutput and are not dependent upon position when using suitablecarburettors so that they can even be used in a steeply inclinedposition.

[0003] In essence, two principles are known for the lubrication oftwo-cycle engines, namely petroil lubrication, in which oil is alreadyadmixed beforehand to the fuel at a mixing ratio of 1:25 to 1:100, andseparate-lubrication, in which oil is pumped from a separate oil tank bymeans of an oil pump into the crank housing or the carburettor nozzle.Both lubricating methods help make it possible to utilise two-cycleengines in any position and also serve to keep the weight of suchengines low.

[0004] Whereas during petroil lubrication the oil must be added to thefuel during each fuelling procedure, it is necessary in theseparate-lubricating process to top up the separate oil tank at regularintervals, as in the case of two-cycle engines there is basically a losslubrication, i.e. there is no oil circulation. Apart from the weight ofthe additional oil tank, the requirement for regular cleaning and alsothe provision of an oil tank indication, safety devices are requiredwhich prevent the engine from starting-up if oil supplies are notsufficient. These devices increase considerably the complexity and theweight of the essentially simple and robust two-cycle engine.

[0005] DE 198 00 904 A1 discloses a combustion engine for a tool,wherein a cam drive for a valve control of the engine is lubricated bymeans of an oil sump. The rotational movement of the toothed wheelswhich are associated with the cam drive serves to generate an oil mistwhich can be guided into a crank housing of the engine where itlubricates the moving parts.

[0006] It is the object of the invention to provide a tool having alubricating system for the combustion engine, wherein the complexity isreduced with respect to lubricating systems which are known from theprior art.

[0007] The inventive solution of the object is provided in claim 1.According to claim 1, the tool is characterised in that the combustionengine can be lubricated by the oil from the housing.

[0008] With respect to the invention, it is assumed that the tool isalready provided with a large quantity of oil, namely for the purpose oflubricating the movement-conversion device, i.e. the transmission in thehousing. In accordance with the invention, this oil is then also usedfor the purpose of lubricating the combustion engine. The oil from thetransmission is consequently consumed over the course of time. Anadditional oil tank as in the case of the known separate-lubricatingprocedure is thus not required nor is it necessary to fill the tool withan oil-fuel mixture.

[0009] In an advantageous manner, the combustion engine can belubricated exclusively with the oil from the housing and the oilquantity provided in the housing is such that it is sufficient for thepurpose of lubricating the movement-conversion device and the combustionengine during a predefined servicing interval. This relieves theoperator of the obligation of checking the oil level at regularintervals, of topping up the oil or supplying a mixture. On thecontrary, topping up the oil into the housing which accommodates themovement-conversion device is performed by a specialist within the scopeof a service inspection which must be carried out at regular intervalsin any case for such highly stressed tools.

[0010] In an advantageous manner, a conveying device is provided for thepurpose of conveying oil from the housing, which surrounds themovement-conversion device, to the combustion engine.

[0011] Since the quantity of oil present is thus limited, it isnecessary to use the oil extremely economically during lubrication ofthe oil-consuming combustion engine.

[0012] Thus, in the case of a particularly advantageous embodiment ofthe invention, the oil which is supplied to the combustion engine isprovided in the form of an oil aerosol, i.e. as oil mist. The oilaerosol is generated in the housing by means of the movement-conversiondevice, in which numerous components move at high speed, whereby the oilprovided in a sump is extensively centrifuged in the form of dropletsand very finely atomised.

[0013] In the case of an advantageous development of the invention, theoil or oil aerosol is collected by means of a collecting device, whichis coupled to the conveying device, in the housing. In order to assistthe collecting procedure, it is expedient if the conveying devicecomprises a suction device. In order to be able to transport the oil tothe combustion engine, it is also advantageous if the conveying devicecomprises a pressure-generating device for the purpose of generating apressure gradient between the housing-side and the combustionengine-side. The pressure-generating device can also serve to pressurisethe oil or the oil aerosol and to inject it at the combustion engine.

[0014] In the case of a particularly advantageous embodiment of theinvention, the pressure-generating device comprises a pressure reservoirwhich is coupled to a combustion chamber of the combustion engine. Uponignition of the air-fuel mixture, there is a sudden increase in thepressure in the combustion engine, wherein some of the high pressurepasses in the form of pressurised exhaust gas from the combustionchamber into the pressure reservoir.

[0015] As already discussed above, the oil or oil aerosol provided mustbe used extremely sparingly, so that the oil supply is sufficient overthe servicing interval. It has proven to be particularly advantageous,if the oil or the oil aerosol can be discharged in a region of a contactsurface between a piston and a cylinder of the combustion engine. Thecontact surface is the region of the combustion engine which intribological terms is loaded to the greatest extent and is thus ingreatest need of lubrication.

[0016] In the case of a particularly advantageous embodiment of theinvention, there is provided an oil outlet which is fed by the conveyingdevice and through which the oil or oil aerosol can be discharged,depending upon the position of the piston in the cylinder, on to arunning surface of the piston shaft and on to a running surface of thecylinder lying opposite the oil outlet. As a consequence, the contactsurface between the piston and the cylinder can be wetted in acontrolled manner with oil or the oil aerosol.

[0017] The conveying device which guides the oil from the housing to thecombustion engine can be controlled in an advantageous manner independence upon the operating state of the engine. For example, it hasbeen shown that the combustion engine does not require lubrication inthe no-load state, whereas in the full-load state there is an increasedrequirement for lubrication. The controllability of the conveying deviceagain considerably enhances the ways of utilising the limited oilsupplies economically.

[0018] This and further advantages and features of the invention will beexplained in detail hereinunder by preferred examples with reference tothe accompanying Figures, in which

[0019]FIG. 1 shows a schematic illustration of the structure of a toolin accordance with the invention;

[0020]FIG. 2 shows a rammer for ground-compaction as an example of atool in accordance with the invention;

[0021]FIG. 3 shows a sectional view of a combustion engine in thedirection of the arrows A in FIG. 2, wherein a piston is positioned inthe bottom dead centre;

[0022]FIG. 4 shows a sectional view as shown in FIG. 3, wherein thepiston is positioned shortly before the top dead centre; and

[0023]FIG. 5 shows a schematic illustration of a pressure-generatingdevice associated with the invention.

[0024]FIG. 1 shows a schematic illustration of the structure of a toolin accordance with the invention, which can be, for example, a hand-heldrammer for ground-compaction purposes as shown in FIG. 2 in the partialsectional illustration. In principle, like components in FIGS. 1 and 2are thus also designated by like reference numerals.

[0025] A cylinder 2 and a piston 3, which can move in the cylinder 2,are disposed in a known manner in a combustion engine 1 which is knownper se and is formed as a two-cycle engine, said piston causing rotationof a crank shaft or a drive shaft 5 by means of a connecting rod 4.

[0026] The drive shaft 5 extends from the housing of the combustionengine 1 into a housing 6 which surrounds a movement-conversion device7. In the case of the rammer as shown in FIG. 2, the movement-conversiondevice 7 comprises a toothed wheel pair 8, a crank drive 9 and astamping piston 11 which is driven by way of a connecting rod 10 andwhich by way of a spring system 12 causes a tube 14, which is associatedwith a stamping base 13, to carry out a stamping movement. Since thisoperating principle has been known for a long time, a further detaileddescription thereof is superfluous. The movement-conversion device 7 issketched in FIG. 1 merely in a schematic manner in the form of a toothedwheel and a frame, as illustrated by dotted lines, which surrounds saidtoothed wheel.

[0027] The movement-conversion device 7 can thus comprise differenttypes of toothed wheel transmissions, crank transmissions, etc. andserves substantially to change the directions of movement, types ofmovement (translatory, rotary; continuous, intermittent, oscillatory,jerky, etc.) and movement speeds.

[0028] The moving components of the movement-conversion device 7 arelubricated with oil. In the case of the example shown in FIG. 2, oil isintroduced into the housing 6 and the movement-conversion device 7 afterremoval of an oil inlet screw 15 and ultimately accumulates in the formof an oil sump at the bottom in the ramming system onmovement-conversion device 7.

[0029] By reason of the high speeds of the moving components of themovement-conversion device 7, oil is extensively centrifuged in acontinuous manner and swirled in the form of large and small droplets.After merely a short period of time, an oil mist consisting of an oilaerosol and made up of extremely fine droplets of oil is formed in thehousing 6 and this wets all of the parts which are to be lubricated.

[0030] In the case of the tool in accordance with the invention, thisoil aerosol, which swirls in the housing 6 is captured and guided to thecombustion engine 1 where it also used for lubrication purposes.

[0031] To this end, the housing 6 is provided with a collecting device17 which constitutes substantially an orifice in the housing 6, intowhich the oil aerosol can issue. At this site, it can be expedient toprovide a filter or a porous, sponge-like material, in order to filterthe oil aerosol or to allow merely the finest oil droplets through, andno particles of dirt which may be present.

[0032] A conveying device 18 serves to guide the oil aerosol from thecollecting device 17 to an oil outlet which is formed as a nozzle 19.The conveying device 18 can be formed in various ways. For example, ithas thus been found that the oil aerosol can also be transported throughhoses having an extremely small diameter over a relatively longdistance. It is thus even sufficient to generate a low pressuregradient, in order to transport oil from the collecting device 17 to thenozzle 19.

[0033] The design of the conveying device 18 depends substantially uponwhether a pressure must be generated at the nozzle 19 and if so at whatlevel. Apart from conveying devices familiar to the person skilled inthe art, it has also proven advantageous to use a pressure-generatingdevice which will be explained hereinunder with reference to FIG. 5.Equally, it can be expedient for oil to be supplied to the conveyingdevice 18 in liquid form and not as an oil aerosol and for the oil to bechanged into an aerosol only by the conveying device 18. Techniquessuitable for this purpose are ones which are known in the field ofink-jet printers (bubble-jet or ink-jet principle). Accordingly, it isalso possible to use, for example, piezo-actuators. Since the oil andoil aerosol quantities to be conveyed are very small, it is alsosufficient to operate the conveying device 18 at minimum conveyingpower.

[0034] In order to render it possible to consume the oil in thecombustion engine 1 in the most economic manner possible, in the case ofthe tool in accordance with the invention only the part of thecombustion engine 1 which is most highly loaded in tribological terms iswetted with oil. This relates to the contact surface between the piston3 and the cylinder 2, in particular the contact surface between pistonrings 20, which are inserted into the piston 3, and the cylinder 2.

[0035] It has proven to be particularly advantageous to wet theso-called piston sleeve, i.e. a piston shaft 21 below the piston rings20, with oil.

[0036]FIGS. 3 and 4 show a sectional view of the combustion engine 1 ofFIG. 2, as seen in the direction of the arrows A.

[0037] In FIG. 3, the piston 3 is at the bottom dead centre, as can beseen from the relative position between the connecting rod 4, the piston3 and the cylinder 2. Accordingly, FIG. 4 shows a position shortlybefore reaching the top dead centre.

[0038] The piston rings 20 are inserted into the piston 3.

[0039] The nozzle 19 which has already been discussed in connection withFIG. 1 and is used as an oil outlet is inserted directed in an inclinedmanner upwards below a cylinder running surface 22, i.e. the innersurface of the cylinder 2. As shown in FIG. 3, oil or the oil aerosolcan be sprayed via the nozzle 19 on to the piston shaft 21, if thepiston 3 is located at the bottom dead centre. Then, namely the outersurface of the piston shaft 21 is freely accessible.

[0040] In contrast, if the piston 3 has moved upwards into the cylinder2, the running surface 22 of the cylinder 2 is exposed and can then bewetted with oil by the nozzle 19. Therefore, it is possible to lubricatesufficiently the most extensively loaded parts of the combustion engine1 with extremely small oil quantities. For example, the previouslydiscussed control of the conveying device 18 can be configured in such amanner that it takes into consideration the respective piston positionand only provides oil at suitable points in time. Naturally, the oil canalso be injected in a continuous manner.

[0041] Instead of using the nozzle 19, it is also possible to use anozzle ring which extends around the periphery of the piston 3. In orderto reduce the consumption of oil still further, the use of so-calledwedge-type rings as piston rings 20 has proven to be advantageous.Moreover, the piston shaft 21 should be provided with an emergencyrunning coating, e.g. consisting of Graphal®.

[0042] The further movable parts of the combustion engine 1 which arenot lubricated at all or not sufficiently lubricated with oil must havelife-time lubrication or consist of suitable materials to withstandrelatively high thermal loading. However, it is to be assumed that smallresidual quantities of oil issue out at the piston 3 and can be used forthe purpose of lubricating the other components. All other slidingsurfaces which require lubricant (for example, crank shaft bearingarrangement) can be equipped with sealed bearings which have life-timelubrication.

[0043] The features described have rendered it possible to improve themixing ratio of oil to petrol to 1:800 which means that the quantity ofoil present in the housing 6 of the tool is sufficient to guarantee thelubrication of both the movement-conversion device 7 and also thecombustion engine 1 during an entire servicing interval. In any event,at the end of the servicing interval, the tool must be inspected by aqualified engineer. This provides the opportunity of replenishing theoil supply in the housing 6.

[0044]FIG. 5 shows one possible inventive way of providing pressurewhich can be utilised by the conveying device 18.

[0045] For this purpose, a combustion chamber 23 which is formed bymeans of the cylinder 2 and the piston 3 is connected to a pressurereservoir 24 by way of a non-return valve 25 and a restrictor 26. Afurther valve 27 is provided for the purpose of emptying the pressurereservoir 24 or for removing the pressure-loaded gas.

[0046] After ignition of the air-fuel mixture in the combustion chamber23, very high pressure is suddenly produced which is used not least tomove the piston 3 downwards and to drive the drive shaft 5. However, aportion of the highly pressurised exhaust gas in the combustion chamber23 passes via the non-return valve 25 into the pressure reservoir 24.Therefore, with each combustion stroke the pressure in the pressurereservoir 24 increases until finally there is a pressure equilibrium atthe non-return valve 25. The pressure in the pressure reservoir 24constitutes an energy medium which can be used for the purpose ofdriving or assisting the conveying device 18.

[0047] In order to limit the pressure in the pressure reservoir 24, atwo-port, two-position directional control valve [not illustrated] canbe used which interrupts further charging-up of the pressure reservoir24 upon achievement of a predetermined pressure. Only when the pressurein the pressure reservoir 24 has fallen below the predefined controlpressure, does the directional control valve render it possible torecharge the pressure reservoir 24. The pressure reservoir 24 does nothave to be configured in the form of a conventional reservoir with amembrane and gas-filling arrangement. The presence of a short line piececan even be sufficient in certain circumstances for the purpose ofstoring and providing sufficient pressure for the conveying device 18.

1. Ground-compaction device, having a combustion engine (1) comprisingat least one cylinder (2) and a piston (3) which can move in thecylinder (2); and in addition a movement-conversion device (7), which isdisposed in a housing (6), for the purpose of converting a rotationalmovement generated by the combustion engine (1) into a working movementfor ground-compaction; wherein the combustion engine (1) is disposed onor in the housing (6); and the housing (6) is filled with oil for thepurpose of lubricating the movementconversion device (7); characterisedin that the combustion engine (1) is lubricated by the oil from thehousing (6) of the movement-conversion device (7), wherein the oilsupplied to the combustion engine (1) is discharged in the region of acontact surface between the piston (3) and the cylinder (2). 2.Ground-compaction device as claimed in claim 1, characterised in thatthe combustion engine (1) is lubricated exclusively with the oil fromthe housing (6).
 3. Ground-compaction device as claimed in claim 1 or 2,characterised in that a conveying device (18) is provided to convey oilfrom the housing (6) to the combustion engine (1).
 4. Ground-compactiondevice as claimed in any of claims 1 to 3, characterised in that thequantity of oil provided in the housing (6) is such that it issufficient to lubricate the movement-conversion device (7) and thecombustion engine (1) during a predefined servicing interval. 5.Ground-compaction device as claimed in any of claims 1 to 4,characterised in that the oil which is supplied to the combustion engine(1) is an oil aerosol.
 6. Ground-compaction device as claimed in claim5, characterised in that the oil aerosol is generated in the housing (6)by means of the movement-conversion device (7).
 7. Ground-compactiondevice as claimed in any of claims 3 to 6, characterised in that thehousing (6) is provided with a collecting device (17), which is coupledto the conveying device (18), for the purpose of collecting the oil oroil aerosol.
 8. Ground-compaction device as claimed in any of claims 3to 7, characterised in that the conveying device (18) comprises asuction device.
 9. Ground-compaction device as claimed in any of claims3 to 8, characterised in that the conveying device (18) comprises apressure-generating device (24 to 27).
 10. Ground-compaction device asclaimed in claim 9, characterised in that the pressure-generating devicecomprises a pressure reservoir (24) which is coupled to a combustionchamber (23) of the combustion engine (1), wherein a pressure isgenerated in the pressure reservoir (24) by means of exhaust gas fromthe combustion chamber (23) during or immediately after a combustionprocedure.
 11. Ground-compaction device as claimed in any of claims 3 to10, characterised in that the conveying device (18) is controlled independence upon an operating state of the combustion engine (1). 12.Ground-compaction device as claimed in any of claims 1 to 11,characterised in that the oil or oil aerosol is discharged on to arunning surface of a piston shaft (21) of the piston (3). 13.Ground-compaction device as claimed in claim 12, characterised in thatin or below a running surface (22) of the cylinder (2) there is providedan oil outlet (19) which is coupled to the conveying device (18) andthrough which the oil or the oil aerosol is discharged on to the runningsurface of the piston shaft (21) and on to the running surface (22) ofthe cylinder (2).